A device of this kind is described in French Patent Application No. 81 19 585, filed on Oct. 19, 1981. The first processing circuit of this device is of a conventional type and is composed mainly of a first amplifier for the signals present on the output electrode of the transducer, a gain compensation device and a display device. The second processing circuit, being connected parallel thereto, itself comprises (a) a second amplifier which is also connected to the output electrode of the transducer; (b) a group of n mutually parallel channels which are connected to the output of this second amplifier and each of which successively comprises a band-pass filter (the group of filters thus provided being such that their pass-bands are consecutive and span approximately the pass-band of the second amplifier) and an envelope detector (itself composed of a rectifier and a low-pass filter having a variable time constant); (c) an arithmetic circuit which is connected to the output of the n channels and which uses the output signals thereof in order to calculate a parameter which is an indicator for the spread of the amplitudes of the signals of central frequency of each channel and which is at the same time directly locally correlated with the mean slope (referred to as the differential ultrasonic attenuation factor) of the curve of the variation of the ultrasonic attenuation as a function of the frequency in the tissues scanned; and (d) a circuit which is connected to the output of this arithmetic circuit and which serves to determine the value of this factor whithin each of the zones bounded by the echos corresponding to the obstacles encountered in the tissues scanned, the output signals of the latter circuit providing modulation of the image presented on the display device.
A device having such a construction is interesting because it enables quantitative information to be derived by local calculation of a parameter which is directly related to the differential ultrasonic attenuation factor, followed by direct display of the values of this factor in A-type or B-type echograms. However, the results obtained by means of this device are affected by measurement noise which limits their accuracy.